Pershing Square Capital Management's William Ackman has had a lot of ideas about what to do with bookseller Borders Group, in which it owns a substantial stake. But Ackman's latest is almost certainly his boldest, a deal that would unite the country's two largest bookstore chains.

Ackman said this week that his hedge fund would help finance a nearly $963 billion takeover of Barnes & Noble by Borders. The former put itself up for sale earlier this year and the latter has struggled in recent years, requiring several infusions of cash from Pershing Square, which owns a 15% stake in Borders but holds warrants and swaps that could increase that total to 42%.

It's not the first time that Ackman has pushed for such a tie-up, having argued in favor of a merger before selling Pershing Square's 11% stake in B&N two years ago. Also in 2008, Ackman argued in favor of a deal between Borders and Amazon.com.